In modern full-duplex teleconferencing systems, subband acoustic echo cancellers are used to cancel reverberant sound incident at the local room microphone, thus to avoid the far end user having to hear echoes of his/her own voice. These devices model the character of the open air sound paths between the microphone and the loudspeaker, by decomposing the wide-band speech signals into several disjoint subbands each associated with an independent adaptive filter; and developing an impulse response function for each subband to emulate the component of the room impulse response contained within each subband.
A subband acoustic echo canceller contains a number of adaptive filter "taps" that are allocated among the subbands. The allocation can be uniform (same number of taps in each band); or nonuniform. The maximum number of feasible taps as summed over all subbands is a known function of the real-time processing capability of the hardware employed in the echo canceller. Given this constraint, it is critical that the feasible number of taps be distributed over all subbands in some kind of optimized filter tap "profile" calculated to remove the maximum possible reverberent acoustic energy. A typical tap profile may, for example, be based on an assessment of maximum echo path compensation capability vs. subband (i. e., frequency) number. This type of tap profile is derived from data reflecting known gross characteristics of typical room acoustic impulse response functions, which takes into account the fact that the magnitude of the response decays with increasing time and increasing frequency. Using this approach, it has been common practice in setting the echo canceller filter tap profiles to simply allocate most of the taps to the lower-most subbands, with the tap allocation in each subband decreasing roughly exponentially with increasing subband number. This allocation "weights" the lower frequencies with more filter taps in those subbands; and is carried out using a table of tap counts stored in the subband acoustic echo canceller.
One shortcoming of this traditional tap allocation and weighting scheme is the absence of any indicia of human perceptual phenomena in the weightings. These phenomena significantly affect the degree to which components of the echo are actually perceived by and annoy the far-end talker.